Is it possible to use multiple desktop environments on same system?

I am using Ubuntu 12.04. I want to try out KDE and other desktop environments.

But I'm not sure if I'll like them enough to switch permanently. I want to be safe and make a transition in a piecemeal fashion.

Is it possible to install KDE on Ubuntu and yet not do away with the default Unity shell?

Could I install multiple desktop environments and switch between them by tweaking some startup configuration and then rebooting?

It is possible, to install KDE run sudo apt-get install --no-install-recommends kubuntu-desktop. After you install it you can switch between Desktop Environments at the login screen.

This also works with most of the common DE's out there.

For Xubuntu (XFCE) sudo apt-get install --no-install-recommends xubuntu-desktop

For Lubuntu (LXDE) sudo apt-get install --no-install-recommends lubuntu-desktop

And Gnome Shell (Gnome 3) sudo apt-get install gnome-core

Theoretically you can have as many of these installed as you want, but I would only use one or two at time to avoid conflicts.

Note: If you plan on moving to a new DE completely, remove the --no-install-recommends from the commands above.

Removing

You can remove any of these with the purge command in apt, like this:

sudo apt-get purge gnome-core

or

sudo apt-get purge kubuntu-desktop

If you want to try Linux Mint's Cinnamon use these commands:

sudo add-apt-repository ppa:gwendal-lebihan-dev/cinnamon-stable  
sudo apt-get update  
sudo apt-get install cinnamon

To uninstall Cinnamon use sudo apt-get purge cinnamon and follow these instructions on removing PPA's.

I have personally run Unity and Kubuntu, Unity and Xubuntu and Unity and Gnome Shell. I have used Cinnamon a little bit too. KDE is by far the most customizable followed by XFCE in Xubuntu. Right now I just use Unity.